1. Field of the Invention
The invention relates generally to GNSS signal processing and more particularly to GNSS carrier phase signal processing for providing GNSS position determination with subscribed precision.
2. Description of the Background Art
The Global Positioning System (GPS) is operated by the United States government for providing free GPS positioning signals to all users around the world. Stand alone GPS receivers can use a coarse/acquisition (C/A) code in these signals for computing unaided positions having typical accuracies of about five meters or about two meters with WAAS. This level of precision is sufficient for many applications including most navigation applications. However, there are positioning applications, such as survey, mapping, machine control and agriculture, where greater precision and/or protection against outlier positions is needed.
Some of these needs are met by differential GPS systems that provide GPS code phase corrections. A GPS receiver that is constructed for differential GPS operation can use the code phase corrections for computing positions having typical accuracies of a few tens of centimeters to one meter. These accuracies are sufficient for most positioning applications. However, a user cannot be altogether confident in the accuracies of stand alone or differential GPS positions because the integrity of the positions is affected by outliers mostly due to multipath. Multipath reflections of the GPS signals can cause outlier errors of meters to tens of meters depending on the extra distances that are traveled by reflected signals.
Real time kinematic (RTK) systems use highly accurate carrier phase measurements of GPS signals in order to provide greater position accuracy and high integrity. A rover GPS receiver that is constructed for RTK operation can determine relative positions to an accuracy of about a centimeter to a few tens of centimeters. These positions have high integrity. It is very unlikely that any of the measurements corresponding to the position contain large multipath errors. Existing GPS RTK services provide RTK carrier phase measurements to the users for a cost that is largely driven by the fixed infrastructure costs for providing the system divided by the number of users. However, some users require precisions better than stand alone GNSS precision but do not require the centimeter precisions of the RTK systems. Some users have the need for the integrity of RTK-based positioning but do not require the full accuracy that it provides.
U.S. Pat. No. 7,292,183 by Bird partially resolves this requirement by providing RTK reference carrier phase data for degraded rover accuracy by dithering (intentionally varying) the reference carrier phases before they are transmitted. This dithering is done with an irreversible process to ensure that the user of the rover can never obtain the high-accuracy position that would be obtained with the pre-degradation carrier phases. An aspect of the U.S. Pat. No. 7,292,183 is that the reference data provides the same selected positional precision to all GPS rovers.